In deep water offshore oil field developments, pipelines are installed to transport crude oil and gas from subsea well-heads to fixed platforms or floating storage facilities.
Crude oil contains many different chemical components and substances, from gases to semi-solid hydrocarbons, such as asphalt and paraffin wax, and frequently also water. Under the high underground pressures and temperatures, the crude oil flows easily in a liquid or gaseous state. When the hot crude oil comes from the reservoir to the ocean floor and enters the pipeline for transportation to the platform, it gets into contact with a cold sea water environment, which at larger depths (1000 ft+305 m) has temperatures of about 40 degrees F. (4–5° C.). Under these conditions the crude oil will cool down rapidly. When cooled down, water and the semi-solid components and/or gases in the crude oil tend to solidify forming hydrate and paraffin deposits on the pipeline wall. Consequently the pipeline cross-section is reduced and the flow capacity is diminished, adversely affecting the oil production. In extreme cases complete stoppage of the pipeline may occur.
To prevent the build-up of paraffin and hydrates, pipeline and flowlines can be insulated on the outside with thermal insulating materials, to reduce heat loss of the flowing crude oil, and to guarantee a required arrival temperature at the separating facility. Successful thermal insulating materials used in offshore pipeline applications include polyurethane foams, and epoxy and urethane based syntactic foams with glass microspheres.
One commonly used form of pipeline insulation consists of a “pipe-in-pipe” (PIP) structure, in which thermal insulation is applied to the surface of a pipeline and the insulation is surrounded by an outer pipe or “casing pipe”. Generally, there is an annulus between the interior surface of the casing pipe and the outer surface of the thermal insulation. A number of insulated pipes (a “pipeline bundle”) may be enclosed in the same casing pipe. Commonly, the thermal insulation comprises a plurality of C-section panels of insulating material secured to the outer surface of the pipeline. Flexible panels are also used, which can be deformed to wrap around the pipeline. It is also common to pressurise the interior of the casing pipe using, for example, nitrogen. This improves the resistance of the casing pipe to collapse under hydrostatic pressure and thus allows the wall thickness of the casing pipe to be reduced. Where the thermal insulation comprises an open-celled foam, the pressurising gas will also penetrate the foam.
In an insulated PIP assembly of this general type, one mechanism for heat transfer between the pipeline and the casing pipe is by convection. This is particularly so in the case of open-cell foam insulating materials. The use of pressurised gas in a PIP system increases convection and hence increases heat transfer between the insulated pipeline and the casing pipe. Where the thermal insulation comprises C-section panels or other discrete panels or the like, gaps usually exist between adjacent panels, typically of the order of ⅛th inch (0.3175 cm). The present inventors have found that the presence of such gaps seriously reduces the effectiveness of the thermal insulation, as a result of convection when the casing pipe is pressurised. This convective heat loss effect is negligible when the gas in the casing is at atmospheric pressure, but becomes increasingly significant with increasing casing pressure. WO99/05447 discloses a deep sea insulated pipe encased by an insulating core, the pipe having a protective outer casing. The insulation being made from microspheres contained in a resin bearing foam.